The present invention relates to improvements in a pattern detecting apparatus used to precisely measure the position of a wafer, a mask or the like in a mask aligner which is employed in semiconductor production.
An example of the pattern detecting apparatus will be explained below with reference to FIG. 1 which shows a reduction projection aligner disclosed in a U.S. Pat. No. 4,380,395 assigned to the present assignee, and which is also used in the present invention. In a reduction projection aligner, a circuit pattern which is formed on a semiconductor wafer 4 in a preceding step, is overlaid with another circuit pattern on a reticle 2, which is to be newly formed on the wafer, through an exposing condenser lens 1. Usually, such processing is repeated successively for a plurality of reticles to form a desired circuit pattern on the semiconductor wafer 4. At each processing, it is required that one of the two circuit patterns is aligned to the other with an accuracy of less than 1 .mu.m. In the reduction projection aligner shown in FIG. 1, the above-mentioned positioning is made in such a manner that the position of a positioning pattern on the semiconductor wafer 4 is detected and a relative movement is made between the wafer 4 and the reticle 2 so that the reticle 2 having the circuit pattern to be newly formed is accurately aligned to the wafer 4.
In more detail, the positioning reference pattern (which is omitted in FIG. 1 but is indicated by reference numeral 4' in FIG. 2) on the semiconductor wafer 4 is locally illuminated by means of a light guide 6. Light reflected from the positioning pattern passes through a reduction lens 3, a through hole 5 provided in a reticle holder 12, the reticle 2 and a magnifying optical system 7, and is then imaged on a movement plane on which a uniaxial movable table 10 provided with a slit 8 is moved, to form an enlarged image of the positioning pattern. At this time, if the illumination light given by the light guide 6 for detecting the positioning pattern is thrown on a wide region around the positioning pattern, a photoresist in the illuminated region is exposed to the light and therefore the fabrication of devices is subjected to great restrictions, even in the case where it is not required to provide a reference pattern on the reticle 2 as in the reduction projection aligner shown in FIG. 1. Therefore, it is required to retrict the light for detecting the positioning pattern by the through hole 5 provided in the reticle holder 12 or a shading pattern 5' (namely, a shading aperture) on the reticle 2 so that the light illuminates a small region on the semiconductor wafer 4, for example, a region having dimensions of 40 .mu.m by 40 .mu.m. In the following explanation, there will be shown only the case where the light for detecting the positioning pattern is restricted by the shading pattern 5' on the reticle 2.
An image plane of the magnifying optical system 8 is scanned by the slit 8, and the intensity of light having passed through the slit 8 is detected and converted into an electric signal by a photomultiplier 9 in accordance with the displacement of the slit 8. At this time, the displacement of the slit 8 is measured by a linear encoder 11.
FIG. 2 shows a relative position at the image plane between the positioning pattern 4' (for example, a linear pattern having a width) on the semiconductor wafer 4 and the shading pattern 5' (for example, a rectangular pattern) on the reticle 2. When the photocomposite image of these patterns is scanned by the slit 8, such a detection signal as shown in FIG. 3 is obtained. In FIG. 3, the displacement X of the slit 8 is plotted as abscissa and the digital value Y, into which the analog output of the photomultiplier 9 indicating the intensity of light having passed through the slit is converted, is plotted as ordinate. The displacement of the slit 8 is measured by the linear encoder 11, and the output data at an i-th reading position provides a digital value Y.sub.i. A signal varying region 22 at a central part of such an output signal corresponds to the positioning pattern 4' on the semiconductor wafer 4. Further, signal varying regions 21 and 23 at the peripheral parts of the region 22 correspond to both ends of the shading pattern 5' on the reticle 2.
In general, the image of the shading pattern 5' is excellent in contrast, and therefore it is easy to accurately detect the position of the image. On the other hand, the image of the positioning pattern 4' is inferior in contrast due to the fact that a photoresist is present on the semiconductor wafer 4. Accordingly, it is required to detect the position of the image of the positioning pattern 4' from such an output signal as shown in FIG. 3, for example, in a manner as disclosed in U.S. Pat. No. 4,115,762. That is, a given position X.sub.i of the slit 8 is taken as a virtual center, and m data on one side of the position X.sub.i are laid upon m data on the other side to calculate a value ##EQU1## A position of the slit 8 which corresponds to the smallest one of many values of Z thus obtained, is used as the position of the center of the positioning pattern 4' on the semiconductor wafer 4.
However, the above-mentioned method has a drawback that it is impossible to accurately detect the position of the positioning pattern 4' on the semiconductor wafer 4 when the positioning pattern 4' on the semiconductor wafer 4 comes close to an end portion of the shading pattern 5' on the reticle 2 as in FIG. 4A which shows a relative position at the image plane between the patterns 4' and 5'. A detection signal obtained at this time is shown in FIG. 4B. In this case, a value Z.sub.1 of Z which is obtained by calculation when the center position X.sub.1 of the signal varying region 22 corresponding to the positioning pattern 4' is used as the virtual center, becomes greater than a value Z.sub.2 which is obtained when a position X.sub.2 shown in FIG. 4B is used as the virtual center (see FIG. 4C). Accordingly, a position of the slit 8 which corresponds to the smallest one of values of Z, cannot be used as the center of the positioning pattern 4' on the semiconductor wafer 4. Therefore, according to the above-mentioned method, a pattern detecting apparatus can detect the positioning pattern only in a central range of about 10 .mu.m width. Accordingly, in the case where the positioning pattern 4' on the semiconductor wafer 4 comes close to an end portion of the shading pattern 5' on the recticle 2 and therefore the position of the positioning pattern 4' departs from the detectable range of the pattern detecting apparatus as in the above-mentioned example, the above-mentioned method cannot detect a correct position of the positioning pattern 4' on the semiconductor wafer 4.